Vygotsky's view on value of peers
But your authors omit a crucial element by glossing the "zone of proximal development" as "the part of knowledge where (a person) still needs a teacher".
Vygotsky knew that learning was social, and confirmed that a child's peers play an indispensable role in enabling learning. He saw the zpd as "those (mental) functions in the process of maturation...the 'buds' or 'flowers' of development rather than the 'fruits'" and specifically as relating to what a child could do "under adult guidance or in collaboration with more capable peers".
Therefore the practice of ability setting deprives students of the opportunity to learn from peers who may be "more capable" and cultivates educational failure.